Candida albicans is both a ubiquitous part of the mammalian commensal flora and the most common systemic fungal pathogen of humans. Systemic disease is the fourth most common nosocomial infection and is associated with a ~40% mortality, due both to the severity of the infection and current inadequacies in diagnosis and treatment. Systemic, or disseminated, candidiasis mostly develops in patients whose innate immune system has been compromised by disease, chemotherapy, or medical intervention (surgery or implanted devices such as catheters). Thus, there is a fine line between Candida as a commensal and Candida as a pathogen and our long-term goal is to understand how this balance is maintained or disrupted to promote one state or the other. It is our premise that strengthening the immune system or weakening the fungus, even slightly, may tip this balance in favor of the patient, thus we have studied the interaction between C. albicans and cells of the innate immune system. From these studies, we have evidence that C. albicans secretes an immunomodulatory compound(s) that inhibits the release of nitric oxide (NO), a key antimicrobial and immunomodulatory compound, from macrophages. We have begun to characterize this inhibitor and have found that it is small, hydrophilic, heat-stable, and is not carbohydrate-based; one aim of this proposal is to identify this compound. In doing so, we have been aided by a series of genomic experiments that have defined the extensive and complex transcriptional response of C. albicans to phagocytosis by macrophages. One of the most surprising findings is an induction of the arginine biosynthesis pathway in its entirety. No other nucleotide or amino acid pathway is induced, making this a specific and unique response. In addition to being an essential amino acid, arginine is also the substrate for production of NO by the inducible Nitric Oxide Synthase (NOS2 or iNOS). Analogs of arginine inhibit iNOS; one such analogue has been shown to greatly reduce the anti-Candida activity of neutrophils. These arginine analogs fit our preliminary chemical characterization of the C. albicans-derived inhibitor. Thus, our central hypothesis is that C. albicans has co- opted the arginine biosynthesis pathway to produce an iNOS inhibitor, related to arginine, that promotes survival and pathogenesis of this organism. The experiments proposed here will test this hypothesis and identify the inhibitory compound. Serious fungal infections, caused mostly by Candida species, are increasingly common and severe. These affect mostly patients already debilitated by other medical treatments or illnesses and suggest that strengthening these patients' immune system would help fight these infections. We present data here that indicates that the fungus itself may be actively impairing the immune system and propose to characterize how this process occurs with the hope of eventually counteracting this ability. [unreadable] [unreadable] [unreadable]